Such energy storage and power generation systems have been known for many years. Major limitations of these systems have resulted from the practical application of what seems to be a simple direct chemical process. Hazardous materials, efficiencies, system size, plugging and clogging, gas formation, "plating out" or precipitation of the materials, membrane diffusion limitations, cost of materials and cost of operation highlight the practical problems. Another limitation of such systems is the loss of power output as the system discharges.
The fundamental chemical process in these systems is characterized by a chemical equation where the reaction proceeds in one direction in the charging of the system and in the opposite direction during the power generation by the system. An example of a redox system is given by the following chemical equation, the term "redox" defining reactions in which a reduction and complementary oxidation occur together. EQU Eq. 1 Cr.sup.2+ Fe.sup.3+ .revreaction.Cr.sup.3+ +Fe.sup.2+
In this system, limitations exist since the chromium is expensive and the chromium and iron, meant to be on either side of a membrane, cross over contaminating the other side. This necessitates frequent reprocessing of the electrolyte. Furthermore, noble metal catalysts are required to promote the reaction. My U.S. Pat. No. 4,069,371 entitled, "Energy Conversion", issued on Jan. 17, 1978, describes a system to continuously counter a rising pH tendency.
Another example of an electrochemical cell for the production of electricity is a zinc-bromine cell in which the overall chemical reaction can be written as follows: EQU Eq. 1a Zn+Br.sub.2 .revreaction.Zn.sup.2+ +2Br.sup.-
The main limitation of this system is the non-uniformity of the zinc deposition onto the electrode which leads to an imbalance when an array of cells is cycled.
U.S. Pat. No. 4,485,154 discloses an electrically rechargeable anionically active energy storage and power delivery system using a sulfide-polysulfide catholyte reaction and an iodide-polyiodide, chloride-chlorine or bromide-bromine anolyte reaction, with recirculating electrolytes.
It has been found that a disadvantage of operating the system of U.S. Pat. No. 4,485,154 using the bromine/sulfide couple is that the pH of the catholyte falls as the system is periodically cycled resulting in H.sup.+ ions diffusing and being transported electrically into the anolyte, resulting also in a lowering of the pH of the anolyte with the attendant formation of H.sub.2 S.
U.S. Pat. No. 4,343,868 discloses a zinc-bromine battery in which the zinc forms the negative electrode and takes part in the electrochemical reactions in the cell. One of the side reactions is the evolution of hydrogen with an attendant loss of hydrogen ions from the negative electrolyte and a rise in pH. Means are provided in U.S. Pat. No. 4,343,868 for the adjustment of the pH of the negative electrolyte in the downwards direction by the generation of hydrogen ions.
An object of this invention is to provide an energy storage and/or power delivery process and apparatus in which the pH changes and/or the changes in concentration of hydroxyl ions in either or both of the chambers of the cell are compensated for by the generation of hydroxyl ions.
It is another object of this invention to provide such a process which can be chemically recharged by replacing or reconstituting one or both electrolytes outside the cell making the cell continuously operable.
It is a further object of this invention to provide economical power generation, that is electrically rechargeable.
It is yet another object of the invention to provide a process with a sufficient power density for practical applications.
Another object of this invention is to provide substantially full power even at low states of charge of the system, i.e. the system power production stays reasonably constant over time maintaining substantially the same output until close to complete discharge.